Aluminum and aluminum alloys have been strengthened by several techniques. One method involves the addition of soluble elements such as magnesium, copper, silicon or zinc that strengthen the crystal structure of the alloy by replacing an aluminum atom in the lattice randomly with an atom of a different element. This is known as solid solution strengthening and leads to modest strength improvements. A second strengthening method is alloying the aluminum metal with elements such as copper, magnesium, silicon or zinc that have solubility in the aluminum crystal structure at elevated temperature. These elements have reduced solubility as the temperature is reduced to room temperature, resulting in precipitation of a second phase containing the added element. By controlling the cooling rate from an elevated temperature, a supersaturated solid solution can be obtained. This supersaturated solution can be manipulated by a combination of temperature and time to allow controlled precipitation in the aluminum crystal structure. This is the most common technique for strengthening aluminum alloys. Alloys such as 2024 aluminum contain copper and magnesium to generate precipitation, 6061 aluminum contain magnesium and silicon that generate precipitation and 7075 aluminum contains zinc, copper and magnesium that generate precipitates. As the use temperature of the alloy increases, the precipitates tend to agglomerate and loose their ability to impede dislocation motion and to impart strength.
Methods of obtaining improved tensile strength in aluminum based alloys have been described in U.S. Pat. No. 2,963,780 to Lyle et al.; U.S. Pat. No. 2,967,351 to Roberts, et al.; and U.S. Pat. No. 3,462,248 to Roberts, et al. The alloys taught by Lyle, et al. and by Roberts, et al. were produced by atomizing liquid metals into finely divided droplets by high velocity gas streams. The droplets were cooled by convective cooling at a rate of approximately 104° C. per second. As a result of this rapid cooling, Lyle, et al. and Roberts, et al. were able to produce alloys containing substantially higher quantities of transition elements than has hither to been possible.
Higher cooling rates using conductive cooling, such as splat quenching and melt spinning, have been employed to produce cooling rates of about 105 to 106° C. per second. Such cooling rates minimize the formation of large intermetallic precipitates, with acicular or blocky morphology, during the solidification of the molten aluminum alloy. Such intermetallic precipitates are responsible for premature tensile instability.
U.S. Pat. No. 4,379,719 to Hilderman, et al. discusses rapidly quenched aluminum alloy powder containing 4 to 12 wt % iron and 1 to 7 wt % cerium or other rare earth metals from the lanthanum series. U.S. Pat. No. 4,647,321 to Adam discusses rapidly quenched aluminum alloy powder containing 5 to 15 wt % iron and 1 to 5 wt % of other transition elements. U.S. Pat. No. 4,347,076 to Ray, et al. discusses high strength aluminum alloys for use at temperatures of about 350° C. that have been produced by rapid solidification techniques. These alloys, however, have low engineering ductility and fracture toughness at room temperature, which precludes their employment in structural applications where a minimum tensile elongation of about 3% is required. U.S. Pat. Nos. 4,828,632; 4,878,967 and 4,879,095 to Adam et al. discuss rapidly solidified aluminum base alloy powder products of Al—Fe—Si—X where X is specifically vanadium or at least one element from the group V, Mn, Cr, Mo, W, Ta or Nb. These techniques have resulted in high strength alloys that generally suffer from low tensile ductility at room temperature. The alloys have very high strength at elevated temperatures and therefore suffer from the lack of workability. The alloys tend to be unstable at higher temperatures and catastrophic precipitate growth occurs rendering the alloys unusable due to reduced strength and induced brittleness.
The use of powder metallurgy routes to produce high strength aluminum has been proposed and has been the subject of considerable research. Powder metallurgy techniques generally offer a way to produce homogenous materials, to control chemical composition and to incorporate dispersion strengthening particles into the alloy. Also, difficult-to-handle alloying elements can at times be more easily introduced by powder metallurgy than ingot melt techniques. The preparation of dispersion strengthened powders having improved properties by a powder metallurgical technique known as mechanical alloying has been disclosed, e.g., in U.S. Pat. No. 3,591,362. Mechanically alloyed materials are characterized by fine grain structure, which is stabilized by uniformly distributed micron sized particles such as oxides and/or carbides. U.S. Pat. Nos. 3,740,210 and 3,816,080 pertain particularly to the preparation of mechanically alloyed dispersion strengthened aluminum. Other aspects of mechanically alloyed aluminum-base alloys have been disclosed in U.S. Pat. Nos. 4,292,079; 4,297,136 and 4,409,038; such as, the requirement to off-gas the blended powder due to hydrogen absorption during the ball-milling operation. In addition to the need for off-gassing, the powder must be handled in a controlled atmosphere because the fresh surface created by the ball milling renders the powder pyrophoric. The rapid oxidation of the fine powder can result in a fire or an explosion. These difficulties make these processes difficult to scale-up and the materials have not been widely used.
In precipitation strengthened aluminum alloys, the precipitates which elevate the strength of such alloys will grow in size, agglomerate and eventually dissolve into the matrix as the temperature is raised, severely degrading the strength of the alloy. The introduction of strong inert nano-meter (10−9 m) sized particles into aluminum is desirable because these particles have similar size as precipitate particles are initially and will inhibit dislocation motion in the aluminum grains. This will result in high strength aluminum. Being inert, the nano-meter sized particles will not react with the aluminum matrix, the strength of the alloy will be relatively unchanged at all temperatures up to the melting temperature. Several techniques have been employed to introduce nano-meter sized particles into aluminum. The high-energy ball mill process described earlier has been used to break-up larger particles of aluminum to generate nano-sized aluminum particles whose strength declines as the temperature rises. This process results in the generation of wide range of particle sizes and has problems with scale-up. U.S. Pat. Nos. 7,297,310 and 7,288,133 and U.S. Pat. No. 8,323,373B2 disclose using the oxide layer that is present on all aluminum powder as the source for nano-sized aluminum oxide particles. These processes require the use of fine aluminum powders in order to have sufficient aluminum oxide present to create a usable composite. As the powder size is reduced to a size where sufficient oxide is present for composite production, the price of the powder becomes too high for commercial processes and is extremely dangerous to handle because of its pyrophoric property. For example, U.S. Pat. No. 8,323,373B2 teaches that the oxide thickness on the aluminum particles is approximately 5 nm regardless of the atomization process. By geometry, one is able to calculate that particles with 30 micron diameter will have an oxide content of 0.1 volume percent with the 5 nm thick oxide layer. The oxide content will increase to 0.15 volume percent for 20 micron particles, to 0.3 volume percent for 10 micron particles and to 0.6 volume percent for 5 micron particles. The aluminum particle size must be reduced to 1 micron in order for the alumina content to become 3 volume percent.
Other techniques that have been evaluated for incorporation of nano-sized ceramic particles into aluminum or other light metals are pressure infiltration and direct mixing of aluminum powder with nano-meter sized particles. The pressure infiltration process involves the production of a reinforcement mat or block. The reinforcement block is placed into a mold and the mold cavity is sealed. Molten aluminum is poured on to the block and a gas pressure is applied to the top surface of the molten aluminum. The pressure forces the molten aluminum between the particles. As the particle size is decreased to the nano-meter size, the pressure needed to cause infiltration becomes too high for normal commercial equipment. The nano-meter particles of aluminum oxide are not naturally wet by molten aluminum so infiltration is only achieved by the use of extremely high pressures.
Fine aluminum oxide powder is a nonconductor of heat or electricity. Static electricity generated by particle movement causes the powder to agglomerate. Because of the static charge, the agglomerates are difficult to break apart. As the particle size is reduced from a micron size to a nano size (10−6 m to 10−9 m) the tendency to tightly agglomerate increases. Several investigators have attempted to blend the nano-meter size particles into commercial aluminum powders using high shear techniques and high-energy ball mill techniques. These attempts resulted in materials with agglomerates at grain boundaries and at prior aluminum particle boundaries. The majority of the nano-meter size particles were contained in the agglomerates and poor mechanical properties were observed.
The Swiss Federal Laboratories for Material Science and Technology (EMPA) in Thun, Switzerland has shown that it is possible to produce spherical nano sized alumina (Al2O3) particles by use of plasma flame equipment. Metal matrix composite (MMC) material developed at EMPA with nano-size reinforcement particles in an aluminum alloy matrix has been shown to be considerably harder than one reinforced with micron sized particles. However, implanting nano-sized alumina particles into aluminum alloy matrices is rather difficult today simply because the alumina particles are so small that transporting them from the plasma reactor where they are made to the interior of the matrix alloy requires very expensive processing. Additionally, the nano alumina particles tend to agglomerate during transport. The segregation of the nanoparticles results in less than anticipated properties in the ultimate metal matrix composite. We must therefore use innovative techniques to introduce nanoparticles into our composites.
Some work at EMPA has reported that it is possible to coat the surface of micron size alumina with nano-sized particles of the same ceramic composition. In the invention described below, a process of coating nano-size particles on micron size spheres of alumina is utilized as a practical means of introducing significant volume fractions of nano-spheres into MMC materials in a cost effective manner, without the special processing noted above. After powder ingot manufacturing and metal working, this is found to result in dispersion of the nano-size particles with the uniformity that is achievable today with micron size particles.
Some work has been done as well at Gamma Technology that has shown that nano-sized alumina particles can be directly coated onto simple micron sized particles of metallic aluminum at room temperature. This vastly simplifies the manufacture the resulting composite by avoiding the need to coat the nano alumina particles on to micron sized alumina particles on the fly in a plasma flame.
While the handling and transport of micron size ceramic particles in industry today can be done economically, handling large volumes of nano-size powders has been very expensive until now. Using micron size particles of either alumina or aluminum as a carrier in order to incorporate the smaller particles into an MMC will drastically lower the price for the composite. This technique will also allow us to uniformly distribute the nanoparticles during subsequent metal working of the composite. This recipe will produce materials with an increase in properties above that of MMC material to which nano alumina particles have not been deliberately added.